As a conventional sealing device for reciprocal movement of the type described above, there is known a sealing device shown, for example, in FIG. 9(a). That is, the shown sealing device is adapted to seal a space between a housing 100 and a shaft 101, which are relatively reciprocally movable in the axial direction thereof, the known sealing device is provided with an annular sealing device body 103 secured to an inner periphery of a shaft bore 102 of the housing 100 and a seal lip integrally provided on the sealing device body 103.
The seal lip 104 has a lip sliding surface to which a second stage projection 106 is formed mainly for the purpose of stabilizing a contacting condition of a first stage projection 105 due to a pressure variation, or the like. Contact angles .alpha.1 and .alpha.2 between the projections 105, 106 and the shaft 101 on atmosphere or air side and contact angles .beta.1 and .beta.2 therebetween on the oil or liquid side are set to .beta.2&gt;.beta.2 and .beta.1.gtoreq..beta.2&gt;.alpha.1 to improve the sealing performance.
However, in the known art described above, since the first stage projection scrapes off oil a, lubrication shortage is caused with respect to the second stage projection 106, or succeeding stage projections, thus increasing the friction force and providing a problem.
Furthermore, when the sealing pressure increases, since the seal lip 104 is pressed against the shaft, a tension force increases and the friction force is accordingly increased, thus compounding the problem.
Furthermore, as shown in FIG. 9(c), there is known a sealing device provided with a seal lip 107 having a lip sliding surface containing a plurality of fine projection sets 108 for reducing the friction.
However, even in the seal lip 107 having such conventional structure of the multi-stage fine projection sets, as shown in FIG. 9(e), it has only been expected to attain a lubricating effect based on the fact that oil is maintained only between the fine projections 109 having the same shapes and positive oil introduction and its effect has not been considered. Contact angles .alpha. and .beta. on the oil-facing side and the atmosphere-facing side of each fine projection 109 are set to .alpha.=.beta., or, as in the conventional example mentioned above, to .beta.2.apprxeq..beta.1.gtoreq..alpha.2, and in a case where the oil is not introduced between the fine projections 109, the essential friction reducing effect is not achieved because of less lubrication.
Further, as shown in FIG. 9(f), the maximum contact pressure at the front end of the fine projection 109 (a in the figure) is considerably large in comparison with a case of no projection (b in the figure), so that abrasion will progress under a use condition having a thin oil film.
Furthermore, when the sealing pressure increases and the pressing force of the seal lip 107 increases, the oil film becomes thin in an amount corresponding to the increasing of the pressing force and, accordingly, the oil introduction between the fine projections 109 is hindered, resulting in further increasing of the friction force.
The present invention was conceived to solve the above problems and its object is to provide a sealing device for accommodating reciprocal motion between two members and being capable of reducing the resultant friction force of the seal lip and improving anti-abrasion performance.